A. Field of the Invention
The present disclosure relates to an organic electroluminescence device, and electronic equipment.
B. Description of Related Art
In the past decade, much progress has been achieved in research of OLEDs, therefore successively leading to applications to full-color display such as television and mobile telephones. The organic electroluminescent device is the light-emitting diodes device in which holes from the anode, also electrons from the cathode are injected into the emissive layer, then wherein holes and electrons are recombined, and excitons are formed. According to the electron spin of the statistical law, singlet excitons and triplet excitons are produced at a ratio of 25%:75%. Thus, in this case, the internal quantum efficiency of fluorescent light emitted by a singlet exciton is limited to 25% because of the nonradiative decay of the spin-forbidden triplet exciton.
On the other hand, the phosphorescence-based OLEDs have been developed to address this issue, where the internal quantum efficiency can be increased to 100% because noble-metal-based organometallic phosphors possess emissive triplet states through performing the intersystem crossing efficiently from the singlet excitons. However, the noble metals are expensive and the abundances of them are very limited, moreover the blue PHOLEDs need to be further investigated to obtain decent device longevity. Against this background, new concepts have been considered for the fluorescence OLEDs with high efficiency utilizing the delayed fluorescence, such as, triplet-triplet annihilation, which is a phenomenon of generating singlet excitons by the fusion of two triplet excitons.
The internal quantum is just theoretically limited to 40%. Efficient OLEDs based on charge-transfer (CT) Cu(I) complexes have also attracted much interest in the last decade. However, high-performance blue OLEDs based on Cu(I) complexes have not been reported, due to the poor device reliability arising from oxidation of metal center. Therefore, in order to achieve further internal quantum efficiency, the organic EL element utilizing a mechanism involving other delayed fluorescence has been investigated, such as, TADF (thermally activated delayed fluorescence, or thermal activation delay fluorescence). The TADF mechanism utilizes the phenomenon that the reverse intersystem crossing from the triplet exciton to the singlet excitons can be realized when using the material with a small energy difference between the singlet level and triplet level (ΔST). The OLEDs involving this TADF mechanism are described by Taiki et al. (Nature, 2012, V. 492 volume, p. 234-238) where carbazolyl dicyanobenzene (CDCB) acts as the TADF luminescent material. However, these conventional OLEDs using TADF luminescent materials suffer from short operation time.